This invention relates to a cellulose triacetate film which is improved in film strength, particularly in practical strength at low temperature, and a process for producing the same.
In general, cellulose triacetate films are produced by the solution casting process. In the solution casting process, cellulose triacetate and plasticizer are dissolved in a solvent mixture of which the principal solvent is methylene chloride and some poor solvents of cellulose triacetate (hereinafter called poor solvent(s)) to obtain a dope, and the dope is fed uniformly from a die continuously on a circulatingly travelling endless support. The solvents are evaporated on the support, and after the dope is solidified, it is stripped from the support. Then, it is dried to obtain the film.
The cellulose triacetate films produced through the above process are excellent in optical properties and mechanical properties, moisture resistance and dimensional stability, and widely utilized as the film base for photographic films.
However, the cellulose triacetate films are insufficient in mechanical strength, such as tear strength, folding endurance, impact strength and flexibility, and troubles occasionally occur, such as tearing, crease, cutout or the like in a film production process, a film exposure operation in camera, etc.
In order to improve the mechanical strength of cellulose triacetate films, various methods have been proposed, such as a method of shrinking the cast film in a shrinkage percentage range from 3 to 20% in the drying process in the solution casting process (Japanese Patent KOKOKU 49-5614), a method of shrinking the cast film similarly in a shrinkage percentage of 20% or more (Japanese Patent KOKOKU 49-4554), and a method of improving film strength, particularly tear strength, by casting a dope to which a poor solvent is added in addition to methylene chloride onto the surface of a band, heating the cast dope layer so as to become in a temperature range from 30 to 42xc2x0 C. within 3 minutes after casting, and keeping the cast dope layer in the temperature range at least for 15 seconds (as disclosed in GB 2013559) (corresponding to Japanese Patent KOKOKU 61-39890).
Moreover, a method of restraining both planar orientation degree and crystallization degree by controlling the surface temperature and shrinkage percentage of a film to a particular range upon production is also proposed in U.S. Pat. No. 5,152,947 (corresponding to Japanese Patent KOKAI 4-1009).
Incidentally, cellulose triacetate films which are widely used for photographic film, etc. become fragile particularly in low temperature environments, and perforations are occasionally broken by torque upon winding the photographic film. All of the above prior art are directed to the improvement in strength at room temperature, but the improvement in strength at room temperature does not always contribute to the prevention of film from breakage at low temperature.
An object of the invention is to provide a cellulose triacetate film which is resistant to breakage even at low temperature.
Another object of the invention is to provide a process for producing the same.
The inventors investigated in order to achieve the above objects, and found that temperature variation of tear strength of cellulose triacetate film varies according to drying conditions of cast films. Then, they further investigated, and found that films having a slope of tear strength variation with temperature of a particular value or less have great strength at low temperature, and the films can be obtained under particular drying conditions. One of the particular drying methods is that, there is an inflexion point in a relation between drying wind temperature and tear strength, and drying is conducted at a temperature slightly lower than the inflexion point.
Thus, the present invention provides a cellulose triacetate film which has achieved the above object, which comprises a cellulose triacetate film having a film thickness from 115 to 135 xcexcm and a Z value of 0.3 or less wherein the Z value is a slope (g/xc2x0 C.) obtained by plotting tear strength against temperature in a range between xe2x88x9220xc2x0 C. and +23xc2x0 C., and a process for producing the same, which comprises casting a dope having a concentration from 15 to 35 wt. % as the sum of cellulose triacetate and other components which will solidify after drying onto an endless support, stripping the cast film from the support, and drying, wherein said drying is conducted using drying wind having a temperature Txc2x0 C. which satisfies T0xe2x88x9215 less than T less than T0, wherein temperature Txc2x0 C. of the drying wind is in an arbitrary volatile component region and the temperature T0xc2x0 C. is at an inflexion point obtained by plotting tear strength W of the cellulose triacetate film after drying at the temperature Txc2x0 C. against drying wind temperature.